1. Field of the Invention
This invention relates to a method and an apparatus for digitally processing an image, wherein intermediate steps of gradation in a continuous tone can be increased by a random process.
2. Description of the Related Art
An image processing system such as an image reading and reproducing system has been widely used in the printing and platemaking industries, for example, for processing image information recorded on an original or subject to be read to produce an original film plate, with an intention of simplifying the entire process and improving the quality of printed images.
In producing the original film plate suitable for a specific purpose by the above conventional image processing system, image processing regarding such as sharpness adjustment, gradation correction, color correction, trimming of an image, is effected on image data, and the processed image data is transmitted to an image output device, in which the original film plate is produced.
The conventional process referred to above has suffered, however, from a problem that in transmitting the processed image data to the image output device, shortage in the gradation steps has often developed in a high-density zone of the image, though 256 steps of gradation can be reproduced by 8 bits that a digitally processed image data may usually provide.
There has also been a problem of a tone jump (a false contour) when the number of bits of the image data is in short, since a smooth relationship between an image signal read from the original and output image data is lost.
Further, an undesirable increase in a memory capacity arises when the number of gradation steps is increased, because a halftone image is formed by comparing the output values with threshold patterns stored in the memory.